1. Field of the Invention
This invention relates to apparatus for converting digital angular data signals into analog sine and cosine components thereof for use as analog resolver input signals.
2. Description of the Prior Art
In many systems such as avionic navigation apparatus it is desirable to produce at one location an indication of angular data measured or established at another location. For example, in an aircraft navigation instrument display system, angular compass data representative of Heading, Heading Select, Course Select, and Bearing are generated at remote locations and displayed to the pilot on a panel mounted indicator such as a horizontal situation indicator or HSI. The remotely generated data may be repeated on the HSI by analog servo loops controlled by resolvers excited by sine and cosine components of the input angle. That is, the sine and cosine component inputs to the resolver stator are used as the position reference for the servomotor loop to position the indicator display member at the angle corresponding to the angle of the input or command data. The rotor of the resolver is mechanically coupled to the indication device and is rotated by the display servomotor to provide a feedback to the servo loop to zero the input when position agreement is achieved.
Conventionally, this data is supplied to the indicator in analog format such as by a three wire synchro data transmission system which is used to control position servo loops through analog electronics and electric motors for mechanically driving the HSI indicator elements.
In many future avionic systems the navigational angular data will be supplied in digital format from, for example, a digital area navigation computer, and in most cases in accordance with ARINC standards. Typical of such data is compass system data. Accordingly, for at least some of these future avionic systems, it will be necessary to provide apparatus for converting the digital angular data into analog sine and cosine components for excitation of the resolver receiver of an analog servosystem.
One method of obtaining the sine and cosine of the angle is performed by addressing a memory in which the sine or cosine for each individual digital angular data word is stored for "look up" purposes. And, the cosine or sine, respectively, may be trigonometrically determined from the aforementioned "look up" of the sine or cosine. In this system, however, as the sine and cosine functions are symmetric and repetitive functions the memory must encompass at least the sine or cosine function for each digital angular data word generated for 90.degree. of data angle. Accordingly, depending upon the resolution or accuracy of the digital angular data word, that is, the number of words used to describe 90.degree. of data, the size of the memory may be economically impractical. For example, for a 12 bit resolution of a digital angular data word the presently available 10 bit sine or cosine digital memory tables would necessarily have to quadruple in size.
Furthermore, a great many methods and techniques have been proposed in the past for converting a digital signal representing angular data into an analog signal representing this data but the present inventor is unaware of any method or technique which provides the accuracy or resolution of the present apparatus with a minimum memory capacity.
Accordingly, there is a need to provide a simple, relatively inexpensive but reliable apparatus which converts digital angular data into analog sine and cosine components for use in driving a 360.degree. rotational device, such as a HSI display while achieving a 12 bit resolution with currently available memory sizes.